Method of fabricating thin film transistor

ABSTRACT

The present invention relates to a method of fabricating a thin film transistor in which a metal silicide line generated from Metal Induced Lateral Crystallization is located at the outside of a channel region. The present invention includes the steps of forming a semiconductor layer on a substrate wherein the semiconductor layer has a first region, a channel region and a second region in order, forming a gate insulating layer/a gate electrode on the channel region, doping the first and the second region heavily with impurity, forming a metal film pattern making the first region a metal-offset, and crystallizing the semiconductor layer by means of applying thermal treatment to the semiconductor layer having the metal film.

BACKGROUND OF THE INVENTION

[0001] 1. Field of Invention

[0002] The present invention relates to a methods of fabricating a thin film transistor in which a metal silicide line generated from Metal Induced Lateral Crystallization is located at the outside of a channel region.

[0003] 2. Discussion of Related Art

[0004] A polycrystalline silicon TFT (Thin Film Transistor) is used rather than an armophous silicon TFT for high resolution and fast operation speed in a liquid crystal display. The development of laser crystallization enables to fabricate polysilicon TFT's on a large-sized glass substrate under the temperature similar to a temperature in a process of fabricating armophous silicon TFT'S. However, the TFT fabricated by laser crystallization requires a long processing time and the accompnied process equipments, which causes difficulties in mass production.

[0005] Armophous silicon under a metal layer turns to be crystalized by thermal treatment in MIC (Metal Induced Crystallization) after a specific metal layer has been formed on an armophous silicon layer. MIC enables low temperature crystallization and needs no equipments of high expenses. In spite of the merit of the low temperature crystallization, MIC causes a metal contamination which deteriorates and changes the intrinsic characteristics of silicon due to the induction of metal into the crystallized film.

[0006] A new crystallization method of MILC [S. W. Lee & S. K. Joo, IEEE Electron Device Lett., 17(4), P.160, (1996)] is proposed. The new method of MILC enables the crystallization of armophous silicon under a low temperature of about 400° C. A crystallization of armophous silicon progresses laterally as the boundary of the silicon crystallized by MIC in MILC. Namely, the crystallization of silicon is induced laterally against the crystallization by MIC.

[0007]FIG. 1A to FIG. 1D show cross-sectional views of fabricating a TFT in which a silicon layer crystallized by MILC is used as a channel region.

[0008] Referring to FIG. 1A, an armophous silicon layer is deposited on an insulated substrate 100 on which a buffer layer has been formed. An active layer 10 is formed by patterning the armophous silicon layer by photolithography. A gate insulating layer 11 and a gate electrode 12 are formed on the active layer 10 by a conventional method.

[0009] Referring to FIG. 1B, a nickel film 13 having a thickness of 20 Å is deposited on the whole surface by sputtering. Accordingly, the active layer on which the gate electrode 12 is not formed is contacted with nickel.

[0010] Referring to FIG. 1C, a source 10S and a drain region 10D which are doped heavily with impurity are formed in the active layer 10 by ion-implantation. A channel region 10C lies between the source 10S and the drain 10D.

[0011] Referring to FIG. 10D, armophous silicon of the active layer 10 is crystallized by applying a thermal treatment of 300 to 500° C. to the substrate 100 after the above step. Consequently, A portion of armophous silicon of the source 10S and drain 10D on which the nickel film has been formed is crystallized by MIC, while the other portion of armophous silicon where the channel 10C has been formed is crystallized by MILC.

[0012]FIG. 2A shows a TEM picture of a nickel-silicide line formed in the middle of the channel, and FIG. 2B shows a layout of a TFT after the crystalization by a conventional method wherein the arrows indicate the directions of crystallization by MILC. As shown in FIG. 2A and FIG. 2B, a Ni-silicide line is formed in the middle of the channel region in the active layer. The thin Ni-silicide precipitates formed in the source and the drain crystallize silicon and move to the channel region. Accordingly, the Ni-silicide precipitates started to move from both ends of the source and the drain meet each other at the middle of the channel region, resulting in a Ni-silicide line. The Ni-silicide line becomes a defect deteriorating device characteristics, more specifically, the field effect mobility and the threshold voltage to lower the electrical characteristics of polysilicon TFT'S.

SUMMARY OF THE INVENTION

[0013] Accordingly, the present invention is directed to a method of fabricating a thin film transistor that substantially obviates obviates one or more of the problems due to limitations and disadvantages of the related art.

[0014] The object of the present invention is to provide a method of fabricating TFT which prevents the metal-silicide from being a defect in the channel region by means of locating the metal-silicide line outside the channel region.

[0015] Another object of the present invention is to provide a method of fabricating TFT which improves the characteristics of the TFT by means of crystallizing the semiconductor layer by MILC and simultaneously locating the metal-silicide lane generated therein outside the channel region.

[0016] Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

[0017] To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention includes the steps of forming a semiconductor layer on a substrate, said semiconductor layer having a first region, a channel region and a second region in order; forming a gate insulating layer/a gate electrode on said channel region; doping said first and said second region heavily with impurity; forming a metal film pattern making said first region a metal-offset; and crystallizing said semiconductor layer by means of applying thermal treatment to said semiconductor layer having said metal film.

[0018] Also, the present invention includes a method of fabricating thin film transistor comprising the steps of forming a semiconductor layer on a substrate, said semiconductor layer having a first region, a channel region and a second region in order, forming a gate insulating layer/a gate electrode on said channel region, forming a metal film pattern to make said first region become a metal-offset, doping said first and said second region heavily with impurity, and crystallizing said semiconductor layer by means of applying thermal treatment to said semiconductor layer having said metal film.

[0019] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

[0020] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiments of the inventing and together with the description serve to explain the principle of the invention.

[0021] In the drawings:

[0022]FIG. 1A to FIG. 1D show cross-sectional views of fabricating a TFT according to a conventional art;

[0023]FIG. 2A shows a TEM picture of a nickel-silicide line formed in the middle of the channel, and FIG. 2B shows a layout of a TFT after the crystalization, according to a conventional method;

[0024]FIG. 3A to FIG. 3D show cross-sectional views of fabricating TFT according to the present invention;

[0025]FIG. 4 shows states of crystallization of TFT according to the present invention;

[0026]FIG. 5 shows the I-V characteristics of the TFT's fabricated by the present invention and the conventional art respectively; and

[0027]FIG. 6 shows the I-V characteristics of the TFT's fabricated by the present invention and the conventional art respectively, under the high drain voltage.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0028] Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

[0029]FIG. 3A to FIG. 3D show cross-sectional views of fabricating TFT according to the present invention wherein a Ni-offset is formed at a portion of the drain or at the source region and the MILC by Ni is applied to an armophous silicon layer.

[0030] Referring to FIG. 3A, an armophous silicon layer is deposited to the thickness of about 1000 Å by LPCVD (Low Pressure Chemical Vapor Deposition) on an insulated substrate 200 on which a buffer layer has been formed. An active layer 20 is formed by patterning the armophous silicon layer by photolithography. An oxide layer for a gate insulating layer is deposited to the thickness of about 1000 Å by ECR (Electron Cyclotron Resonance)— plasma oxidation and ECR-PECVD (Electron Cyclotron Resonance— Plasma Enhanced Chemical Vapor Deposition), and then a Mo layer for a gate electrode is deposited to the thickness of 2000 Å by sputtering.

[0031] A gate electrode 22 and a gate insulating layer 21 is formed by patterning the Mo layer and the oxide layer with photolithography. Automatically, a channel region 20C and a first region 20-1 and a second region 20-2 are defined in the active layer 20, wherein the channel region 20C lies between the first and the second region 20-1 and 20-2.

[0032] Referring to FIG. 3B, photoresist is coated on the whole surface. Then, a photoresist pattern PR is formed by a selective exposure and development. The photoresist pattern PR, as shown in the drawing, is defined to cover a portion of the second region 20-2 in the active layer close go the gate. Namely, the photoresist pattern is defined asymmetrically with reference to the gate. A Ni film 23 as an inducing metal for crystallization by MIC is deposited to the thickness of 20 to 200 Å by sputtering.

[0033] Referring to FIG. 3C, the photoresist pattern PR is removed by lift-off to elimenate the portion of the Ni film which is on the photoresist pattern PR. Accordingly, an inner portion of the second region 20-2, i.e. the second region 20-2 near the gate becomes a Ni-offset region.

[0034] A source region 20S and a drain region 20D are formed in the active layer 20 by doping heavily the whole surface of the substrate with n-typed impurities. In this case, the gate becomes a doping mask against the channel region and a portion of the drain region becomes a Ni-offset drain region.

[0035] Referring to FIG. 3D, the active layer of armophous silicon turns to be crystallized by a thermal treatment on the substrate under the temperature between 300 and 500° C. As shown in the drawing, a portion of armophous silicon where Ni lies is crystallized by MIC, while the other portions of armophous silicon which are the channel region not contacted with Ni and the Ni-offset drain region are crystallized by MILC. A boundary of the crystallized silicon generated by MIC becomes new seeds for MILC by which silicon in the channel region and the Ni-offset drain region is crystallized laterally. In this case, the crystallized silicons grown from both ends become to meet in the Ni-offset drain region to form a Ni-silicide line. Consequently, the Ni-silicide line locate outside the channel region.

[0036]FIG. 4 shows states of crystallization of TFT according to the present invention.

[0037] Referring to FIG. 4, a gate electrode 22 is on an armophous silicon layer 20 of an active layer, on which a photoresist pattern PR is located toward the drain region. Accordingly, a portion of the armophous silicon layer on which the gate electrode and the photoresist pattern are located becomes a Ni-offset provide that a Ni film is deposited thereon.

[0038] When a thermal treatment is applied to the substrate, a portion of the armophous silicon contacted with Ni turns to be crystallized by MIC on account of the induction of Ni, while the other portion of the armophous silicon not contacted with Ni turns to be crystallized by MILC. In MILC, the crystalliztion of silicon progresses laterally by means of having the boundary of the silicon having been crystallized by MIC as new seeds. In this case, the crystallized silicons having been grown from the ends of the source and the drain respectively meet each other at the drain region of the armophous silicon layer where a Ni-silicide line is to be formed.

[0039] Hence, the Ni-silicide line is prevented from the channel region. As is mentioned in the above explanation, Ni-silicide in the channel region should be avoided since the defects in the channel region deteriorate the field effect mobility and the threshold voltage. The Ni-silicide line lowers the device characteristics.

[0040] In the foregoing embodiment, Ni is used as an inducing metal. Instead, one of or mixture of metal such as Pd, Ti, Ag, Au, Al, Sn, Sb, Cu, Co, Cr, Mo, Tr, Ru, Rh, Cd, Pt or the like can be used as the inducing material. In the present invention, a portion of the drain region becomes the Ni-offset or a metal-offset region. With the same method, a portion of the source region can become the Ni-offset region or a metal-offset region to crystallize armophous silicon. The only difference is to form a photoresist pattern to cover the portion of the source region near the gate. In this case, a portion of the source region not contacted with Ni is to be called as a Ni-offset source region in the following explanation, and a Ni-silicide line or a metal-silicide line will be located in the Ni-offset source region or a metal-offset source region.

[0041] Moreover in the above embodiment, as shown in FIG. 3B and FIG. 3C, the source and the drain region are formed by an impurity doping process after a Ni pattern to define a Ni offset on the armophous silicon layer, which can be processed in a reversed manner. Namely, a TFT is fabricated by doping the disclosed armophous silicon layer with impurities, by forming a source and a drain region, by forming a Ni pattern to define a Ni offset on the armophous silicon layer, by crystallizing the armophous silicon layer and by the rest steps for fabrication.

[0042]FIG. 5 shows the I-V characteristics of n-typed TFT's fabricated by the present invention and the conventional art respectively.

[0043] The characteristics, related to threshold voltage and on-current, of a TFT according to the present invention is superior to that of a TFT of the conventional art. The prevention of the defect which interferes with the charge flow in the channel region improves the on-current characteristics since there occurs no Ni-silicide line.

[0044] The following table indicates the device characteristics of an n-typed TFT wherein a threshold voltage is given by 1V and a drain current is given by 0.1 μA. Each field effect mobility is the value caculated in the linear area under the drain voltage of 0.1V. The maximum on/off current ratios are attained by the gate voltage between −10V to 20V and the drain voltage of 1V. TABLE Ni-offset Ni-offset Conventional types of TFT's source drain TFT threshold voltage (V) 2.5 2.5 3.3 slopes of subthreshold 0.98 0.93 0.97 voltage (V/dec) field effect 120 82 60 mobility (cm²/V) maximum on/off current 2E6 2E6 1.3E6 ratios

[0045] As shown in the table, the physical characteristics of the TFT having the Ni-offset in the source or the drain according to the present invention are superior to those of the other TFT according to a conventional art.

[0046]FIG. 6 shows the transfer characteristics and the I-V characteristics of the TFT's fabricated by the present invention and the conventional art respectively, under the high drain currents and the high drain voltage of 10V.

[0047] Referring to FIG. 6, the leakage current of the TFT having the Ni-offset outside the drain region indicates the minimum value, which means that the degree of the leakage current depends on the location of the Ni-offset.

[0048] In the above exemplary explanation, defining the Ni-offset or a metal-offset on the source region or the drain region to have the Ni-silicide line or a metal-silicide line located in the drain or the source region, the present invention is also applied to the method of having the Ni-silicide line located or a metal-silicide line outside the channel region. Namely, the Ni film is just formed on the drain region or the source region to define the Ni offset on the whole source or the drain region. By the way, the present invention is further applied to any crystallization of other semiconductor material.

[0049] The present invention improves field effect mobility and threshold voltage by MILC and reduces leakage current by means of having the metal-silicide line generated from the crystallization of semiconductor outside the channel region. Accordingly, polycrystalline TFT's which are very effective and also highly reproductive are fabricated.

[0050] It will be apparent to those skilled in the art that various modifications and variations can be made in a method of fabricating a thin film transistor of the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. 

What is claimed is:
 1. A method of fabricating thin film transistor comprising the steps of: forming a semiconductor layer on a substrate, said semiconductor layer having a first region, a channel region and a second region in order; forming a gate insulating layer/a gate electrode on said channel region; doping said first and said second region heavily with impurity; forming a metal film pattern making said first region a metal-offset; and crystallizing said semiconductor layer by means of applying thermal treatment to said semiconductor layer having said metal film.
 2. The method of fabricating thin film transistor according to claim 1 , wherein said metal-offset of said metal film pattern is defined near said gate electrode in said first region.
 3. The method of fabricating thin film transistor according to claim 1 , wherein said metal-offset of said metal film pattern is defined in said first region.
 4. The method of fabricating thin film transistor according to claim 1 , said step of forming a metal film pattern further comprising the steps of: forming a photoresist pattern covering a certain part of said first region; forming a metal film on said photoresist pattern and a whole disclosed surface of said substrate; and removing said photoresist pattern by lift-off.
 5. The method of fabricating thin film transistor according to claim 4 , wherein said photoresist pattern is formed to cover a portion of said first region close to said gate electrode.
 6. The method of fabricating thin film transistor according to claim 4 , wherein said photoresist pattern is formed to cover a whole surface of said first region.
 7. The method of fabricating thin film transistor according to claim 1 , wherein said metal film pattern is formed by using one of or mixture of metal such as Pd, Ti, Ag, Au, Al, Sn, Sb, Cu, Co, Cr, Mo, Tr, Ru, Rh, Cd, Pt or the like.
 8. The method of fabricating thin film transistor according to claim 1 to claim 4 , wherein said metal film pattern is formed to the thickness of 20 to 200 Å.
 9. The method of fabricating thin film transistor according to claim 1 , wherein said thermal treatment is processed under the temperature of 300 to 500° C.
 10. A method of fabricating thin film transistor comprising the steps of: forming a semiconductor layer on a substrate, said semiconductor layer having a first region, a channel region and a second region in order; forming a gate insulating layer/a gate electrode on said channel region; forming a metal film pattern to make said first region become a metal-offset; doping said first and said second region heavily with impurity; and crystallizing said semiconductor layer by means of applying thermal treatment to said semiconductor layer having said metal film. 